1. Field of the Invention
The present invention relates to a transparent screen including a combination of a Fresnel optical component for bending image light radiated from a projector toward an observer and an optical diffusing component for scattering the image light, and to a projection display unit installing the transparent screen.
2. Description of Related Art
Unlike a CRT (Cathode Ray Tube) or PDP (Plasma Display Panel), a projection display unit is a non-light emitting type display unit.
As shown in FIG. 25, a conventional projection display unit has a projector 1 including an illuminating optical system 2 for illuminating a light valve 3, the light valve 3 for generating image light with adjusting a quantity of light in response to an image signal, and a projection optical system 4 for projecting an image onto a transparent screen 6 by radiating the image light generated by the light valve 3.
The projection display unit is classified into rear projection that projects the image light onto the transparent screen 6 from its back with respect to an observer, and front projection that projects the image light onto the transparent screen 6 from this side of the observer.
As shown in FIG. 25, the transparent screen 6 used for the rear-projection display unit includes a Fresnel lens screen 7 which is a Fresnel optical component for bending the image light radiated from the projector 1 toward the observer side, and an optical diffusing component 10 for providing a diverging angle to the image light to spread it.
A Fresnel lens 9 constituting the Fresnel lens screen 7 is usually fabricated in such a manner as to have a period smaller than a projection pixel (such as 1/10 of the pixel). Thus, the Fresnel lens 9 becomes very thin (such as 100 microns in thickness including its prism section).
To support the very thin Fresnel lens 9, the Fresnel lens screen 7 has a Fresnel lens substrate 8.
The Fresnel lens substrate 8 is usually made of glass or a resin such as PMMA, MS and PC, and the Fresnel lens 9 is usually formed on the Fresnel lens substrate 8 directly.
Although FIG. 25 shows an example of a light-output side Fresnel lens having the Fresnel lens 9 formed on the light-output side of the Fresnel lens screen 7, this is not essential. For example, as shown in FIG. 26, a light-input side Fresnel lens is also possible which has the Fresnel lens 9 formed on the light-input side of the Fresnel lens screen 7.
The optical diffusing component 10, which includes at least a lens element 11 and an optical diffusion sheet 12, is generally referred to as a lenticular screen.
Although the example of FIG. 25 includes a reflecting mirror 5, an example without the reflecting mirror 5 as shown in FIG. 26 is also possible.
When observing the image light radiated from the projector 1 through the transparent screen 6, an observer will recognize innumerable light and dark spots (glare) at random. This is because the unevenness on the surface of the optical diffusing component 10 and its internal refractive index distribution, phase distribution and transmittance distribution have fluctuations greater than the wavelength of the image light.
The light and dark spots, which are generally referred to as speckle or scintillation, present a problem of image degradation.
The following Patent Document 1 discloses an example that disposes a speckle reduction micro-lenticular lens on a light source side of the Fresnel lens screen 7 to reduce the speckle.
By disposing the speckle reduction micro-lenticular lens, the image light radiated from the projector 1 is provided with a diverging angle θ v. During the propagation over the distance between the Fresnel lens screen 7 and the lens element 11, the image light provided with the diverging angle θ v spreads in proportion to the propagation distance t0, and illuminates the optical diffusion sheet 12 which is closer to the light source than a black stripe layer is.
In this way, the image light radiated from the projector 1 is spread through the speckle reduction micro-lenticular lens during the propagation over the distance between the Fresnel lens screen 7 and the lens element 11. When the propagation distance t0 is short, the diverging angle θ v the speckle reduction micro-lenticular lens provides must be increased.
When the Fresnel lens 9 is formed on the light-input side of the Fresnel lens screen 7 as shown in FIG. 26, the speckle reduction micro-lenticular lens cannot be disposed on the light-input side of the Fresnel lens screen 7. Accordingly, the speckle reduction micro-lenticular lens must be disposed on the light-output side of the Fresnel lens screen 7.
The thickness of the lens element 11 is several hundred microns at most, and the greater part of the propagation distance t0 consists of the thickness of the Fresnel lens substrate 8 of the Fresnel lens screen 7. Thus, if the propagation distance t0 is reduced by a factor of 10 from 3 millimeters to 300 microns, 10 times the diverging angle θ v is required.
Although the image light radiated from the projector 1 spreads in proportion to the diverging angle θ v, the energy of the entire image light is conserved according to the energy conservation law, which presents a problem of dimming the image light by an amount of spreading.
It is generally preferable for the screen of the projector to be “bright and wide in the viewing angle”. However, since there is a tradeoff between them according to the energy conservation law, the image light cannot be spread irrespective of the relationship.
As described above, the point of origin of the speckle phenomenon is the unevenness on the surface of the optical diffusing component 10 or the fluctuations of its internal refractive index distribution, phase distribution and transmittance distribution. Thus, the image light transmitted through the fluctuation structure, that is, the wavefront of the image light, is disturbed through the fluctuation structure, as a result of which the light and dark spots are formed.
The characteristic length of the fluctuation structure (the period of fluctuations when fluctuating regularly, for example) becomes a problem.
Patent Document 1: Japanese patent application laid-open No. 2004-171011 (Paragraph No. [0040] to [0051] and FIG. 11)